Many drug- or chemical-induced toxicities to various organs as lung, kidney, and liver occur by metabolism of the parent compound to reactive electrophilic metabolites. Electrophilic metabolites have also been implicated in chemical carcinogenesis. A role for the nucleophile glutathione (GSH) in detoxification has been established with the reactive metabolites of bromobenzene and acetaminophen, but not with other toxicities known to be mediated through electrophilic metabolites. The role of GSH transferase in detoxification of reactive metabolites is uncertain. A simple in vitro assay has been designed to study the possible role of GSH and GSH transferase in the detoxification of reactive metabolites. In this assay the reactive metabolites are generated enzymatically and the detoxification with nucleophiles such as glutathione, cysteine and methionine studied by direct addition to the incubation mixture. The effect of glutathione transferases was determined by addition of these enzymes to the incubation mixtures plus GSH. It has been determined that at 500 micro-M GSH the reactive metabolite of acetaminophen is very effectively detoxified nonenzymatically. At 50 micro-M GSH, however, the reactive metabolite is not effectively detoxified, but after addition of GSH transferase the reactive metabolite is completely detoxified. A similar role for GSH transferase in the detoxification of the reactive metabolites of furosemide, bromobenzene and naphthalene has been established.